When cutting paper, plastic sheet materials, paper bags, plastic bags, etc., a safety cutting device may be used. A regular safety cutting device is generally comprised of a cutting blade, and a plastic shell covering on the cutting blade. The plastic shell has a narrow slot of width about within 0.1 cm to 0.9 cm into which soft thin material is inserted and cut by the cutting edge of the cutting blade. Because the slot does not allow the finger to pass, the safety cutting device is safe to use. However, because the insertion slot perpendicularly extends from the cutting edge of the cutting blade, the cutting effect of the safety cutting device is not satisfactory when cutting a soft, resilient material. Furthermore, because the plastic shell includes two symmetrical shells fastened together to hold the cutting blade therebetween, the manufacturing cost of this type of safety cutting device is high.